August 2, 2017
Pump Vibration – Even Cavitation Isn’t Always Cavitation!
By Stan Riddle
Most people who work on, or around pumps are familiar with the term “cavitation”. It is often used as a catch all phrase for any flow-related vibration on pumps, and is often reported by vibration analysts when:
- Pump noise level is high,
- Pump vibration is erratic, with lots of “noise” in the spectrum.
But there are a handful of flow-related vibration problems on pumps, and only one of those is truly cavitation. But all of them have similar vibration characteristics, and all of them can cause inefficiency of the pumping system, and pump damage, if severe.
Let’s look at the most common reasons for flow-related pump vibration.
CAVITATION – True cavitation occurs when “cavities”, low pressure vapor bubbles, form in the liquid on the suction side of the pump. This can be from:
- Clogged suction strainers, or plugging on the suction side.
- Poor piping design.
- Pump running too far to the left of the pump curve. This is becoming more common with the popularity of VFD’s.
When these low pressure “bubbles” get into the impeller section, pressure balances, and these bubbles implode, often onto the impeller surface.
This can be quite damaging to the impeller and pump housing. Cavitation is often quite loud, and typically explained as “it sounds like gravel is going through the pump!”.
AERATION, OR AIR ENTRAPMENT – Air entrapment happens when bubbles are in the liquid before reaching the impeller. This can happen when:
- The liquid falls into an open tank system, or is aerated near the pump inlet, due to the liquid level lower than the top of the pipe inlet.
- De-foaming agents are not used (or cannot be used)
- Liquid is near its boiling point, such as in a condensate pump.
While this problem is not always as damaging (or as loud) as cavitation, it can certainly damage the impeller if left unchecked.
RE-CIRCULATION – Re-circulation is most often caused by low flow.
Two types of re-circulation problems can occur:
- Suction Re-circulation – reversal of flow at the impeller eye, causing low pressure “bubbles” to form, very much like cavitation. And just as damaging to the suction side of the impeller.
- Discharge Re-circulation – reversal of flow at the discharge side of the pump. Like suction re-circulation, but damaging to the pressure side of the impeller.
Vibration analysis is good at detecting flow-related pump problems, but distinguishing which of these problems is present is difficult, if not impossible, to determine by vibration alone. An easy method to determine which of these problems is taking place is to slowly throttle the discharge valve closed, if operations will permit.
As the discharge valve is being throttled closed:
- If noise and vibration get better – expect cavitation.
- If noise and vibration stay the same – expect air entrapment.
- If noise and vibration get worse – expect re-circulation.
Throttling the discharge valve may seem like an easy fix, but it is merely a diagnosis method. If any of these problems are occurring, there is either an operational or design problem, and it should be addressed by engineering.